1. Field of the Invention
The present invention is generally related to an interface circuit system, and more particularly to a sampling interface circuit for providing the functions of signal sample and hold between the circuits with a different working voltage. And the interface circuit system could protect the circuits with a lower working voltage during the high voltage operation.
2. Description of the Prior Art
When a signal is transmitted from a circuit with a higher working voltage to a circuit with a lower working voltage, an interface circuit is necessary existed between the two circuits to weaken the signal within the voltage accepting range of the circuit with a lower working voltage. For example, the working voltage of a general optical pick-up head is at 5 volts, and the working voltage of an optical disk driver (CD-ROM) control chip is at 3.3 volts. If the writing voltage output by optical pick-up head is directly input into the CD-ROM control chip, the input end oxide layer of 3.3 volts process will collapse and the chip will become permanently damaged after long time stress.
A general CD-ROM control chip 100 as shown in FIG. 1, the optical pick-up head has a voltage input 101, the voltage input 101 could be either a writing voltage (between 3.3 volts and 5 volts) or reading voltage (between 1.4 volts and 2.8 volts). Using the voltage divider composed of the first voltage divider resistor 102 and the second voltage divider resistor 103 to weaken the writing voltage under 3.3 volts, that is the voltage accepting range of a CD-ROM control chip. The input end oxide layer of CD-ROM control chip 100 could avoid collapsing, because the writing voltage is controlled under 3 volts by the voltage divider. The reading voltage produced by the voltage input 101 uses the input end of a CD-ROM control chip such as the switched operation amplifier (SOP) 104 to sample and hold signal to produce an voltage output 105 for the internal part of CD-ROM control chip.
The optical pick-up head has to supply additional current for the path “I” produced by the voltage divider, which consumes additional power and causes thermal noise. If the resistance of the resistor 102 and resistor 103 are larger and then the settling time of the input end of SOP is larger so that the sampling time reduces. The voltage divider weakens not only the writing voltage that is not for sampling but also the reading voltage that is for sampling. If the weakened reading voltage combines with the back end noise, then the signal noise rate (SNR) will be reduced. Besides, two resistors cover a large area of a chip.